1. Field of the Invention
This invention teaches an improved method for processing data received by paired seismic sensors, the sensors being of different genera. More specifically, the method relates to suppression of water-column reverberation interference that is sometimes seen during marine seismic surveys in relatively shallow water.
2. Description of Relevant Art
During the course of a marine seismic survey, a long string of sonic sensors is towed through the water. Periodically, an acoustic source is caused to insonify the subsurface earth layers thereby to generate a reflected seismic wavefield that is detected by the sensors and converted to electrical signals which are transmitted through the cable to the ship. The signals are recorded and later processed for the purpose of displaying the topography of the targeted subsurface earth layers.
In a marine environment, the reflected wavefield makes itself manifest as a variation in hydrodynamic pressure or as a variation in particle velocity. The sensors may be of different genera. That is, they may be designed to measure pressure variation (hydrophones) or particle velocity (geophones). One or the other or both types of sensor may be installed on the same string depending upon the problem to be overcome in the field. Less commonly, accelerometers may be used.
In relatively shallow water on the order of 25-200 feet, so-called bay cables are used. Here, the sensor string is emplaced directly upon the water bottom instead of being continuously towed behind a ship as is done in deep water. Customarily, gimbal-mounted geophones are the sensors of choice for measuring the particle velocity of the sea floor. Hundreds or even thousands of sensors may be deployed, the electrical outputs of the sensors being multiplexed into suitable data recorders.
FIG. 1 shows a ship 10 for handling bottom cable 12 resting on the water bottom 16 beneath a body of water 14. The water surface 18 forms a reflective air/water interface as is well known. The water bottom usually is also a good reflector depending upon the acoustic impedance. Velocity sensor (geophone) 20 and pressure sensor (hydrophone) 22 are co-located on the bottom and are coupled to separate electrical channels in cable 12 through which their signals are sent to archival storage and processing equipment (not shown) on ship 10. For simplicity, only two sensors are shown. Periodically, source 13 generates an acoustic wavefield 15 that propagates into earth 17 whence it is reflected from subsurface strata to return as a reflected wavefield such as 24. Source 13 may be fired by ship 10 or by a separate shooting boat (not shown).
An upcoming reflected compressional wavefield arrival 24 strikes geophone 20 on the bottom and by industry standard, generates a positive-going electrical impulse as shown at 26, FIG. 2. The reflected event continues upward to strike the air/water interface 18 whence it is reflected back downward, after a 180.degree. phase reversal, as a ghost reflection to strike the geophone from the top. Normally, a compressional pulse applied to the top of a geophone produces a negative-going electrical impulse, but because of the phase reversal at the water surface, the first ghost reflection creates a second positive-going electrical impulse 30, FIG. 2, plotted with respect to an arbitrary amplitude scale. Depending upon the acoustic impedance of the water bottom and the water-surface smoothness, wavefields may bounce (reverberate) back and forth betwixt surface and bottom many times much like the multiple reflections seen in the mirrors on opposite walls of a barber shop. The second ghost reflection or multiple 32 is a negative pulse. Subsequent multiples are of alternately positive and negative polarity. Interfering multiple reflections wreak havoc with the recorded data. Reverberation effects have been observed in moisture laden regions such as low-tide beach sand and quicksand.
A hydrophone 22 sees an upcoming reflected compressional wavefield arrival as pulse of compression. Again by industry standard, the hydrophone converts a compressional pulse to a positive-going electrical impulse such as 36, FIG. 3, plotted to the same arbitrary amplitude scale as FIG. 2. The first ghost reflection from the surface travels downwardly as a rarefaction pulse which hydrophone 22 sees as a negative-going electrical impulse 40. The second ghost reflection 42 due to the second bounce between the bottom and the surface will be positive. Subsequent multiples will alternately exhibit negative and positive signatures.
The time interval, .tau., between pulses is the product of twice the apparent water depth multiplied by the water slowness (1/v). For normal incidence as at 28 and 38 (FIG. 1), the apparent depth is the true water depth. Otherwise as for wavefield 44 and path 45, the apparent depth is the product of twice the water depth and the water slowness divided by the cosine of the angle of incidence, .alpha..
Thus, whereas the geophone sees the first reflected arrival and the first ghost arrival as electrical impulses of the same polarity, the hydrophone sees the first reflected arrival and the first ghost arrival as electrical impulses of opposite polarity. Therefore, by using two co-located sensors of different genera and combining their signatures, one should be able to constructively reinforce the first arrival energy and to destructively cancel the subsequent reverberant multiples. A second benefit emerges in that the random noise as seen by the different sensor types is not necessarily the same or correlatable.
A successful merging of the hydrophone and geophone signals requires that the signals from one of the sensor types be scaled to fit the signal from the other type of sensor. Their transfer functions must be matched in phase, amplitude, frequency and damping.
The most straight-forward scaling method would be to demultiplex the sets of recorded seismic data from the two sensors, remove instrumental gain effects, and amplitude-balance the signals using identical amplitude recovery functions for both data sets. The amplitude ratio between the respective first arrivals is the scale factor. However, the presence of noise and other disturbances make that method over-simplistic.
E. M. Hall, Jr. in U.S. Pat. No. 4,253,164, issued Feb. 24, 1981 and assigned to the assignee of this invention, teaches an electrical network for matching the transfer function of an accelerometer or a hydrophone to that of a geophone.
U.S. Pat. No. 5,163,028, issued Nov. 10, 1992 to F. J. Barr teaches an algorithm for deterministically computing a transfer function for matching a hydrophone to a geophone and for compensating for coupling imperfections of the sensors to the water bottom. The method requires use of a special calibration wavefield that is generated from a shot located directly above selected co-located sensors. The calibration shot is generally fired just prior to the beginning a seismic survey.
In U.S. Pat. No. 4,979,150, issued Dec. 18, 1990, also to F. J. Barr, there is taught a computer program for deriving a scale factor for use with a hydrophone co-located with a geophone for attenuating water-column reverberations. The sensors may be positioned at a point in the water above the water bottom or they can be located on the water bottom. The scale factor which derives from the acoustical impedance of the water or the water bottom material can be calculated either deterministically or statistically. In the former case, use is made of a calibration shot fired directly above the sensors as with the previous reference. The statistical methods have no specific requirement for positioning of shot and sensors as is required for the deterministic method. Statistical methods involve iteratively computing ratios between various combinations of auto- and cross-correlation functions of the wavetrains recorded by the respective sensors. Corrections may be introduced for wavefield directivity based upon raypath angularities.
Adaptive noise filtering, a concept that will be introduced later, is explained in Adaptive Noise Canceling: Principles and Applications by Bernard Widrow et al., published in the Proceeding of the IEEE, v. 63, n. 12, December, 1975, pp.1692-1716.
Objections to the above known methods for computing the scale factor involve first, the problem of random noise. In particular, velocity sensors (geophones) are quite noisy relative to hydrophones. With respect to the statistical methods, the geophone noise necessarily contaminates the results of the correlation processes. Second, in the iterative process of determining the scale factor from the cross- or auto-correlation ratios, there is no suggestion of a coefficient of convergence to pin-point the best-fitting scale factor. Furthermore, simple summation of the data from the two sensors as taught by Barr can decrease the signal-to-noise ratio of the summed result relative to that of the hydrophone.
Repeated reference will be made to "water velocity" or "velocity signature". Unless otherwise qualified, the term "velocity" is a short-hand term that means the particle velocity of a medium (earth or water) caused by the passage therethrough of a seismic wavefield. If the word velocity is used in any other sense, it will be so qualified. The terms pressure signature, noise signature and velocity signature refer to the quantitative variation of the amplitude level of the respective parameters as function of time. The collective term "noise signature" includes any and all undesired signals whereas "pressure" and "velocity" signatures are the sought-after seismic signals useful for exploration.
There is a need for an improved method for statistically determining the scale factor between the impulse responses of a hydrophone and a geophone that will not be distorted by noise. Further, there is a need for a method for positively defining a coefficient of convergence for identifying the optimum scale factor and for lessening the impact of geophone noise on the summed data.